Patent GB-350998-A (GB)
Title: Improvements in apparatus for recording sounds upon wax or other like discs or blanks
IPC: C01B33/32
Filed: 1930-03-10
Abstract: 350,998. Moving-coil recorders and microphones. BLUMLEIN, A. D., 57, Earl's Court Square, and HOLMAN, H. E., 64A, The Chase, Clapham Common, both in London. March 10, 1930, Nos. 7809 and 14042. [Class 40 (iv).] A moving-coil recorder, microphone, or other instrument, connected to an operating circuit having an impedance higher than that of the coil, is damped by shunting it with a network of impedances whose total impedance is low at frequencies near the resonant frequency of the mechanical system, and high at frequencies remote from resonance. When the shunt consists of, or is, at frequencies close to resonance, equivalent to, an inductance, resistance and capacity in series, the ratio between the magnitude of these components is the same as the ratio between the magnitudes of the effective mass, damping and compliance of the mechanical system in the region of resonance when the moving coil is closed by this shunt. Circuit arrangements. The shunt of inductance, resistance and capacity in series across the moving coil 1 of a microphone, Fig. 1, requires a high capacity unless a coil of high impedance can be used, and a step-up transformer 2, Fig. 2, may be introduced to increase the apparent impedance of the coil. The shunt of Fig. 2 has an impedance comparable with that of the coil at low frequencies, and to compensate for the increase in impedance of the coil at high frequencies a resistance and capacity is connected in parallel with the inductance of the shunt. Resonance between the valve capacity and the secondary of the transformer 3 is controlled by a resistance 4 across the transformer secondary. The shunt of Fig. 3 is suitable for a sound-recorder, where the low-frequency response is depreciated to prevent excessive amplitude on the record, and therefore the impedance of the shunt below resonance is not required to rise. The condenser in parallel tunes the recorder and its shunt to resonance at a high frequency where its impedance is matched with the amplifier output. In the shunt of Fig. 4, a monitoring circuit is connected across a resistance 15 shunted by a condenser 16. The condenser 9 may be considered as tuning the moving coil or as causing the impedance of the shunt to rise rapidly at high frequency and then to tune the moving coil. The inductance 8 produces a cut-off at a frequency well above the working range to remove the effect of chance high-frequency resonances. The main network consists of inductances 10, 13, resistances 12, 14, and a condenser 11. At low frequencies it is equivalent to inductances 10, 13 in series with resistance 14. In the region of resonance, the capacity 11 and resistance 12 delay a rise in impedance and at high frequencies the network is equivalent to the inductance 10 in series with a resistance 12 and is raised to a very high impedance by capacity 9. The capacity 5, inductance 6, and resistance 7 form a rejector circuit which modifies the characteristic of the recorder. This damping system may also be applied to a moving coil instrument optically recording sound. Construction of microphone. The microphone, Fig. 8, has a magnetic system comprising an inner polepiece r<1> of cobalt iron, rear plate q<1>, casing p<1>, and the outer polepiece h<1>. The air gap is correctly dimensioned by a spacing ring t<1> of brass or cadmium bronze, which is a force fit on the central polepiece r<1> and a good fit in a groove in the outer polepiece h<1>. The inner turns of the winding s<1> are preferably of thinner gauge wire than the outer turns. The moving coil d<1> is formed of thin aluminium wire wound on a former e<1> which has a longitudinal saw-cut to prevent short-circuited turns and may be insulated with a coating of aluminium hydroxide. The former e<1> is riveted to a diaphragm consisting of a layer of Balsa wood a<1> preferably covered on each side by a thin sheet b<1> of aluminium. Alternatively the diaphragm may be a flat sheet of aluminium or other light aLoy with cores of similar material attached to the back to add rigidity. The mounting is preferably an extended annulus c<1> of the back aluminium layer and passes through the centre of gravity of the suspended system. The annulus c<1> is preferably thinned by chemical action, e.g. dissolving by caustic soda, and is tensioned by a ridge n<1> of a front plate m<1> formed with a grille o<1>. The mounting annulus c<1> is clamped at its edge between the holder g<1> and a ring l<1>. The front plate m<1> is formed to prevent sound waves falling on the diaphragm mount, and holes in the former and plugs w<1> of cottonwool prevent excessive air damping. The leads to the moving coil are taken through a passage v<1> to terminals on the holder g<1> or at any other convenient point. Construction of sound-recorder. The soundrecorder, Figs. 5, 6, and 7, has a magnetic system consisting of the polepieces f of a magnet (not shown) at right-angles to a transformer core a with a bulbous portion b. The moving coil d consists preferably of a single, rectangular turn of aluminium, thickened at its ends, and cut from a solid block and moves in the gap g between the polepieces f and the bulbous core b. Currents are induced in it by the coils c of the transformer core a, b. The coil is pivoted about a vertical axis by a shaft e bearing against knife edges under the action of springs h attached to the shaft and passing through holes in the knife edges. The neutral position of the coil is controlled by springs m attached to screws n, and may be adjustable to tune to the frequency of resonance. The shaft e may be integral with the coil or may pass through holes in the thickened portion of the coil and a clearance hole in the core b. The stylus arm i is clamped to the shaft and the stylus k is carried on one arm. A balance weight l serves to balance the stylus arm both statically and dynamically.